Field of the Invention
This invention generally relates to a heat exchanger adapted to be used in a vapor compression system. More specifically, this invention relates to a heat exchanger having a prescribed arrangement of a tube bundle for preventing a vapor flow velocity from exceeding a prescribed level.
Background Information
Vapor compression refrigeration has been the most commonly used method for air-conditioning of large buildings or the like. Conventional vapor compression refrigeration systems are typically provided with an evaporator, which is a heat exchanger that allows the refrigerant to evaporate from liquid to gas while absorbing heat from liquid to be cooled passing through the evaporator. One type of evaporator includes a tube bundle having a plurality of horizontally extending heat transfer tubes through which the liquid to be cooled is circulated, and the tube bundle is housed inside a cylindrical shell. There are several known methods for evaporating the refrigerant in this type of evaporator. In a flooded evaporator, the shell is filled with liquid refrigerant and the heat transfer tubes are immersed in a pool of the liquid refrigerant so that the liquid refrigerant boils and/or evaporates as vapor. In a falling film evaporator, liquid refrigerant is deposited onto exterior surfaces of the heat transfer tubes from above so that a layer or a thin film of the liquid refrigerant is formed along the exterior surfaces of the heat transfer tubes. Heat from walls of the heat transfer tubes is transferred via convection and/or conduction through the liquid film to the vapor-liquid interface where part of the liquid refrigerant evaporates, and thus, heat is removed from the water flowing inside of the heat transfer tubes. The liquid refrigerant that does not evaporate falls vertically from the heat transfer tube at an upper position toward the heat transfer tube at a lower position by force of gravity. There is also a hybrid falling film evaporator, in which the liquid refrigerant is deposited on the exterior surfaces of some of the heat transfer tubes in the tube bundle and the other heat transfer tubes in the tube bundle are immersed in the liquid refrigerant that has been collected at the bottom portion of the shell.
Although the flooded evaporators exhibit high heat transfer performance, the flooded evaporators require a considerable amount of refrigerant because the heat transfer tubes are immersed in a pool of the liquid refrigerant. With recent development of new and high-cost refrigerant having a much lower global warming potential (such as R1234ze or R1234yf), it is desirable to reduce the refrigerant charge in the evaporator. The main advantage of the falling film evaporators is that the refrigerant charge can be reduced while ensuring good heat transfer performance. Therefore, the falling film evaporators have a significant potential to replace the flooded evaporators in large refrigeration systems. However, there are several technical challenges associated with the efficient operation of the falling film evaporator.
One of the challenges is managing vapor flow within the tube bundle of a falling film evaporator. In general, a portion of the liquid refrigerant that vaporized significantly expands in volume in all directions, causing cross flow or travel by the vaporized refrigerant in a transverse direction. This cross flow disrupts the vertical flow of the liquid refrigerant, which increases a risk of the lower tubes receiving insufficient wetting, causing significantly reduced heat transfer performance. Another challenge is preventing entrained liquid droplets from being carried over from the evaporator to the compressor. The compressor can be damaged if the vaporized refrigerant contains entrained liquid droplets.
U.S. Pat. No. 6,293,112 discloses a falling film evaporator in which the tubes of the tube bundle are arranged to form vapor lanes extending in a transverse direction to control the velocity of cross flow of the refrigerant vapor created interior of the tube bundle.
U.S. Pat. No. 7,849,710 discloses a falling film evaporator that includes a hood disposed over the tube bundle. The hood forces the flow of vapor refrigerant to move downward, thereby preventing cross flow of the vapor refrigerant inside the hood. Also, the abrupt directional change of the vapor refrigerant flow caused by the hood results in removal of a great proportion of entrained liquid droplets from the vapor refrigerant flow.